This invention involves a neutral gray colored glass that has a luminous transmittance that makes it highly desirable for use in forward vision areas of a vehicle, such as a windshield and front door windows. In particular, the glass has a luminous transmittance of 60% or greater and preferably 70% or greater. In addition, the glass of the present invention generally exhibits lower infrared and total solar energy transmittance comparable to typical green glasses used in automotive applications to reduce heat gain in the interior of the enclosure. The glass is also compatible with flat glass manufacturing methods.
Various heat-absorbing glass substrates are known in the art. The primary colorant in typical green tinted automotive glasses is iron, which is usually present in both the Fe.sub.2 O.sub.3 and FeO forms. As is conventional, the total amount of iron present in a glass is expressed herein as Fe.sub.2 O.sub.3, regardless of the form actually present. Typical green tinted automotive glass has about 0.5 percent by weight total iron, with the ratio of FeO to total iron being about 0.25.
Some glasses, e.g. U.S. Reissue Pat. No. 25,312 to Duncan et al., produce a gray color in the glass by including nickel oxide as a colorant. However, inclusion of nickel containing materials must be carefully controlled because the presence of nickel during the melting process sometimes leads to the formation of nickel sulfide stones in the glass. Additional potential problems faced when using nickel include haze formation on the glass surface due to reduction of the nickel in the tin bath and change in the glass color when it is heat treated.
To avoid this problem, nickel-free colored glasses containing iron oxide, cobalt oxide, and selenium were developed as disclosed in U.S. Pat. No. 3,296,004 to Duncan et al., U.S. Pat. No. 3,723,142 to Kato et al. and British Patent Specification 1,331,492 to Bamford. In U.S. Pat. No. 4,104,076 to Pons, instead of nickel, Cr.sub.2 O.sub.3 or UO.sub.2 are used in combination with iron oxide, cobalt oxide, and selenium to produce gray glass. A more recent, nickel-free version of gray glass is disclosed in U.S. Pat. No. 5,023,210 to Krumwiede et al. which uses iron oxide, cobalt oxide, selenium, and chromic oxide as colorants.
Many of the commercially available gray glasses are too dark to be used in the forward vision area of a vehicle. In addition, the lighter gray glasses exhibit solar performance properties inferior to those of conventional green automotive glass resulting in elevated interior vehicle temperatures which adversely affect occupant comfort. It would be desirable to have a neutral gray glass that may be used for the vision area of a vehicle, have acceptable solar performance properties and further that is compatible with commercial flat glass manufacturing techniques.